To obtain vegetable fats and oils from oil-containing fruits and seeds both continuous and discontinuous processes are used.
According to the kind of natural product before the actual oil recovery process is performed, a husking or shelling can be necessary since the quality of the oil can be considerably reduced by shell-specific material (e.g. wax) which can appear in the oil on extraction with the solvent.
The conventional pressing frequently employed with fat-rich fruits and seeds is very cost intensive and has lost considerable importance although in a single pressing process a residual fat content of 4% in the pressed residue is attainable. Today processes for continuously recovering oil usually comprise a combination of pressing and extraction with a solvent. First the oil-containing natural materials are subjected to a preliminary pressing to a fat content of about 25% and then extracted to a residual content of 1%. With oil-bearing seeds whose fat content is about 20%, direct extraction is performed without preliminary pressing.
With many plant raw materials a preliminary mechanical treatment involving comminuting or grinding up into smaller pieces is necessary to destroy as much as possible of the storage tissue to attain a high yield. Usually the seeds are subjected to a pressing process after a conditioning. They are moistened and preheated in a unit provided expressly for this purpose.
By this preliminary treatment the oil flows more readily (with a lower viscosity) and separates better on pressing. Moreover a further coagulation of the protein is attained and undesirable enzyme systems and microorganisms are inactivated at temperatures over 80.degree.. With cotton seeds there is also a disinfecting action of the conditioning.
Continuously operating straining screw presses, whose operating principles can be found in the appropriate literature, also are used in the processes described in European Patent 0 129 739 and European Open Patent Application 0 187 877. They are generally used currently in processing heavy oil-bearing seeds and fruits. Working comparisons have clearly shown that in principle a certain screw design or structure only yields optimum results with seed sorting. Different crops require different processing conditions which forces a considerable compromise between quality and operating requirements.
As an extraction medium an industrial grade n-hexane fraction with a boiling range between 55.degree. and 70.degree. C. is generally currently used. For a few special cases, however, other solvents are used. Moreover processes are known, e.g. from German Open Patent Application 2 127 596 and the cited European Patent Document 0 187 877 A1, in which the fat or oil is extracted with supercritical gases.
Solvent separation occurs from the crude oil comparatively easily at temperatures under 100.degree. C. and in vacuum. The residual content of oil in the solvent is under the current analytical detection limit. The solvent is driven out from the solid extraction residue usually with steam.
Despite many attempts it has not been possible to optimize the combination of pressing and solvent extraction in continuous processing and also it has not been possible to obtain satisfactory end product processing in a given plant or unit for a variety of oil-bearing seeds and fruits.